1. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
X.Artru, R.Chehab, M.Chevallier (IPNL, France)
O.Dadoun, P.Lepercq, F.Poirier, A.Variola (LAL, France)
V.M.Strakhovenko (BINP, Russia)
T.Kamitani, T.Omori, T.Suwada, J.Urakawa (KEK, Japan)
T.Takahashi (Hiroshima)
G.Pei, C.Xu (IHEP, Beijing)
L.Rinolfi, A.Vivoli (CERN)
Presented by R.Chehab
R.Chehab/LCWS10/Beijing

2. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
INTRODUCTION
There is a strong interest in having a performing positron source for the future linear colliders.
Performance means: high intensity, 6D emittance matched for the DR injection, and low energy deposition and PEDD
For unpolarized positrons , use of conventional sources (high intensity incident beam, thick amorphous targets) leads to some inconvenients regarding the emittance of e+ beam and a rather high energy deposition and PEDD.
Remedy? multiple-target system (A) or rotating targets (B)
Or hybrid sources using channeling: our choice
R.Chehab/LCWS10/Beijing

3. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
A solution: the multi-target system (A)
From J.Sheppard (NLC)
R.Chehab/LCWS10/Beijing

4. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The rotating target (B): from Ian Bailey
R.Chehab/LCWS10/Beijing

5. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
R.Chehab, V.M.Strakhovenko, A.Variola
The photons from the crystal are impinging on the amorphous target; the e- and e+ can be swept off or partially sent to the amorphous target. Distance L is taken 1.5 to 4 m
R.Chehab/LCWS10/Beijing

6. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The hybrid e+ source using channeling
High energy e- (some GeV) impinging at glancing angles to the main axes of an oriented crystal radiate a large number of soft photons (some tens of MeV, typically) ; this is much more than with ordinary bremsstrahlung in amorphous targets of the same material and thickness; works like an atomic undulator
Successful experiments at CERN (WA103) and KEK
If we separate the photon radiator from the e+ converter and use only the photons (and eventually part of the e+, e-) coming out from the crystal, we may lower considerably the level of PEDD. A practical consequence is the possibility in some cases of using 1-target system, only. Studies on conventional e+ sources at KEK are also carried out to minimize the effects of heating (PEDD,..)
=> Choice of such hybrid system for unpolarized e+ of CLIC
R.Chehab/LCWS10/Beijing

7. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
IS SUCH SYSTEM APPLICABLE FOR ILC ?
The main problem: very high beam intensity which could destroy the targets. To avoid this difficulty, we may think about a modification of the beam structure before the target to lower the number of particles per pulse which is one of the main parameters for the PEDD. The separation between pulses is determined in order to allow the thermal shock wave to damp. After e+ production capture and acceleration the DR restores the nominal time distribution.
Two solutions are considered:
- the 300 Hz (T.Omori, KEK)
- the 45kHz/5Hz (A.Variola, LAL)
R.Chehab/LCWS10/Beijing

8. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
We present here some simulations concerning the hybrid positron source with results on the yield, emittance, energy distribution, capture,..
Two incident energies will be considered: 8 and 10 GeV. The crystal thickness will be 1mm and the amorphous target thickness will be taken between 6 and 10 mm. The distance between the crystal-radiator and the amorphous-converter will be chosen between 2 and 3 meters to allow enough place for a sweeping magnet.
A presentation of the two solutions for lowering the PEDD with the results on the calculations of the PEDD and the associated mechanical stresses in the targets will follow.
The simulations use the results of Strakhovenko’s code with GEANT4 (O.Dadoun)
R.Chehab/LCWS10/Beijing

9. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
THE PHOTON BEAM
Energy spectrum and transverse distribution E-=8 GeV
R.Chehab/LCWS10/Beijing

10. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The photon beam
Energy spectrum, transverse distribution E-=10 GeV
R.Chehab/LCWS10/Beijing
Dimension in cm

11. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The positron beam [E-= 10 GeV]
Transverse emittance distribution (at target (blue) and at AMD end (red))
R.Chehab/LCWS10/Beijing

12. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The positron beam [E-=10 GeV]
Energy spectrum (at target, AMD)
The AMD acts as a low energy filter for the positron beam
R.Chehab/LCWS10/Beijing

13. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
DEPOSITED ENERGY
The incident e- beam is at 10 GeV: the total deposited energy in a 8 mm target is: ~520 MeV/e e- have been simulated
R.Chehab/LCWS10/Beijing

14. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
ENERGY DEPOSITION DENSITY: E- = 10 GeV
L(Xtal)=1mm; L(amorphous)= 10mm: distance= 2 m
PEDD 2.7 GeV/cm3/e- ; e- rms radius: 2.5 mm
R.Chehab/LCWS10/Beijing

15. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
ENERGY DEPOSITION DENSITY: E-=10 GeV
L(Xtal)= 1mm; L(Amorphous)=8 mm; distance=2 m
PEDD 2.11 GeV/cm3/e- ; e- rms radius: 2.5 mm
R.Chehab/LCWS10/Beijing

16. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The positron beam: the yield (tam=8 mm)
Target AMD end Accel (150 MeV)
(with Br)
E-=8 GeV
E-=10GeV
The distance between the two targets is 2 meters
The solenoid at B=0.5 Tesla extends over the accelerated part
The accepted yields present enough margin to realize 1e+/e- at IP.
NB: a rough calculation with simple focusing formulation gave smaller values of accepted yields at AMD (-20%)
R.Chehab/LCWS10/Beijing

17. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
A FACTOR OF MERIT FOR e+ SOURCES: h+/PEDD
The ratio: [accepted yield at AMD end/PEDD] is represented; the accepted yield is at AMD end; the amorphous thickness is indicated on the curves. The PEDD is in GeV/cm3/e-
R.Chehab/LCWS10/Beijing

18. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
ENERGY DEPOSITED, THERMAL GRADIENT, PEDD AND SHOCK WAVES (a semi-quantitative approach)
The high intensity incident electron pulses are depositing a large amount of power in the targets provoking temperature rise. The target material expands but due to the mass inertia, there will be a delay leading pressure waves to cross the material provoking stresses. These waves move with the sound velocity,
v= [E/r]1/2
Where E is the Young modulus and r the density.
The time needed to the wave to travel from the outer surface to the centre is ts = d/v. For target radii of 1 cm, that represents ~ ms . If the pulse width tp > ts , small amount of energy is going in the target in ts and the shock is limited.
R.Chehab/LCWS10/Beijing

19. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
ENERGY DEPOSITED, THERMAL GRADIENT PEDD AND SHOCK WAVES
After the breaking of the SLC e+ target analyses and tests have been undertaken (LLNL, LANL, SLAC). Particularly, the timescales of the different phenomena were determined. It came out that the stress level due to the temperature gradient was lower than the instantaneous shock (see Vinod Bharawadj).
The timescale of the shock wave is of some ms ; so if we spread out the initial ILC pulse in order to have short pulses separated by intervals of microsecond level, a solution can be available.
R.Chehab/LCWS10/Beijing

20. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
1- THE 300 Hz scheme (KEK)
R.Chehab/LCWS10/Beijing

21. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The 300 Hz scheme
R.Chehab/LCWS10/Beijing

22. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The 300 Hz scheme
Referring to the timescale presented above and considering a PEDD of 0.33 J/g per ILC bunch of e-, we have (see T.Omori/ POSIPOL 2009):
- a PEDD of 44 J/g for 132 bunches (triplet)
- a PEDD of 14 J/g for 44 bunches (mini-train)
Between 2 successive mini-trains: 100 ns.
Between two successive triplets: ~3.3 ms, enough to damp the shock wave.
R.Chehab/LCWS10/Beijing

23. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
2- The 45kHz/5Hz solution (LAL) ; mpulse: ms
R.Chehab/LCWS10/Beijing

24. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
The 45kHz/5Hz solution
The PEDD per bunch is of 0.33 J/g ; it is of 20 J/g for a micropulse.
The micropulse duration (22.14 ms) is much larger than the time needed for the shock wave to travel from the outer surface to the centre (~ 3 ms, for a target having r=L ~1cm)
Such situation where tp > ts is favourable => see P.Sievers/CERN
The first micropulse is filling the damping ring, and after ~half the ring period, the second micropulse is filling the ring with a [22.14 ms/2 – (0.369ms/44)] delay
R.Chehab/LCWS10/Beijing

25. AN HYBRID POSITRION SOURCE USING CHANNELING FOR ILC
STRESSES IN THE AMORPHOUS TARGET
For a target having a 10 mm radius and 8 mm thickness and a beam micropulse duration of ms , the stress is given by (see P.Sievers, CERN Note LABII/BT/74-2,1974):
smax= EaTo.[2L/nt0]
Where E, Young modulus; a, thermal expansion coefficient; To, target temperature; L, target thickness; t0, pulse duration; n, the sound velocity. For the W target, we get for a temperature of 1000°C:
smax= 72 MPa
We note the reduction factor of [2L/nt0] due to t0>ts where ts is the time put by the shock wave to cross the target.
This is a rough calculation; similar calculation for SLC target gave a result with a factor 2 in excess wrt finite element calculation (W.Stein et al. PAC 2001).
R.Chehab/LCWS10/Beijing

26. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
THE TARGET FOR THE 45kHz/5Hz: SOLUTION
The 5 W targets are distributed
On a wheel rotating at 5r/sec.
As 520 MeV are deposited by e-
in a 8 mm target [E-=10 GeV]
Each target receives a deposited
energy of: 1.7 J/bunch
and 100 J/mpulse and 4.4 kJ
per second. The target width
is ~5cms on a wheel
of 1 m radius. The g beam
spot of about r=3 mm rms
crosses the target width
in 1.5 ms. The beam impact
on the target is not at the same place from the 1st to
the last mpulse in the 1.5 ms duration.The rotation speed of the target is 31.4 m/s. This is a quite challenging assumption. The “melting density” is about 190 J/g) for W.
target
W =31.4 rad/sec; 300 rpm
R.Chehab/LCWS10/Beijing

27. R.Chehab/LCWS10/Beijing
AN HYBRID POSITRON..
Deposited energy on the rotating target
5 cms
The impinging g beam has a lateral extension of ~ 3mm rms. When the target is rotating successive mpulses are smearing partially one over the other. The distance between the peaks (corresponding to 22 ms) to overcome the smearing is ~10mm => 0.3 ms (v=31.4 m/s). That means that 10 mpulses are concerned. As the smearing is partial, we take a coefficient of 0.5; that corresponds to a complete smearing of 5 mpulses => i.e. 500 Joules. With the volume concerned (1x1x0.8 cm3) that correponds to ~32 J/g.
R.Chehab/LCWS10/Beijing

28. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
AVERAGE ENERGY DENSITY & PEDD [KEK]
The triplet has 132
bunches and deposit
220 Joules in a 8 mm
amorphous target
(E-=8 GeV). The average
Energy density is about 14 J/g
The PEDD is ~ 44 J/g, with a
Nominal PEDD/bunch of 0.33 J/g/bunch
The separation between 2 successive triplets (3 ms) is largely enough to damp the shock wave.
R.Chehab/LCWS10/Beijing

29. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
AVERAGE ENERGY DENSITY & PEDD [LAL]
11 ms
22 ms
The micropulse has 60 bunches; the energy is 100 Joules; the average energy density is 7 J/g for E-= 10 GeV and an amorphous thickness of 8 mm. The PEDD is 20 J/g for a nominal PEDD of 0.33 J/g/bunch
R.Chehab/LCWS10/Beijing

30. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
COMPARISONS BASELINE/HYBRID
Rate of deposited energy:
# For the baseline (EPAC08), the ILC bunch is carrying 10J with 1013 photons. 8 % of this energy is deposited in the target.
# For the hybrid E-=10 GeV, and 8 mm thick amorphous target, the number of photons from the crystal per ILC bunch is: 4.8x1011 g and the deposited energy [1.7 J] represents 10 % of the incident energy (17 J)
Energy Density:
# For ILC baseline, the energy density is of 40 J/g (Ti)
With a wheel rotating at 100 m/s
# For the Hybrid/LAL source, the energy density is 32 J/g (W) for a wheel rotating at 31.4 m/s
R.Chehab/LCWS10/Beijing

31. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
THE UNDULATOR CASE (I.Bailey)
The targets are on a wheel
1 meter diameter rotating
At 2000 rpm and with
tangential velocity of 100 m/s
The energy deposited in the
target is 0.8 J/bunch
Photon beam spot is ~1-2 mm
radius. The average energy
density is, then, of 40 J/g for
the rotating target.
R.Chehab/LCWS10/Beijing

32. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
Some comparisons concerning PEDD, Shock waves,…
We recall that the PEDD/e- in a 8 mm amorphous target submitted to g emitted from 1mm crystal under 10 GeV e- is of ~2 GeV/cm3
For the ILC the average deposited power in 8 mm target is ~20kW
1/Solution A (KEK):
For each triplet ( ~1ms): PEDD 44 J/g :2.6x1012
tp =1 ms < ts =3 ms
The pulse width is smaller than the time needed by the shock wave to travel from the surface to the centre for a target r=L=1cm
2/ Solution B (LAL)
For each micropulse (~22.14 ms): PEDD 20J/g: 1.2x1012
tp = ms > ts = 3 ms
Time needed for the shock wave to travel from the outer surface to centre
R.Chehab/LCWS10/Beijing

33. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
SOME COMPARISONS…
Rotation speed of the target wheel
Baseline LAL scheme KEK scheme
100 m/s m/s m/s
R.Chehab/LCWS10/Beijing

34. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
PRELIMINARY CONCLUSIONS
We have presented an approach for using an hybrid positron source for ILC. The advantages of such a system is to lower the PEDD values on the converter, using only the photons from channeled electrons in a thin crystal. The simulations done are showing that:
# modifying the ILC macropulse time structure before the target, creating a set of micropulses (20or 44) it was possible to lower strongly the energy deposited in a pulse and hence the corresponding PEDD. The choice of the micropulse duration and of the separation between successive micropulses is also essential to allow the shock waves associated to the thermal pulses to damp enough. That seems possible. The nominal time structure is restored with the DR. The modified time structure is compatible with the needed damping times.
# to absorb the average power a rotating target seems necessary but with lower velocity than with the baseline solution. An accurate study of that rotating target is underway considering both options KEK & LAL.
R.Chehab/LCWS10/Beijing

35. AN HYBRID POSITRON SOURCE USING CHANNELING FOR ILC
PRELIMINARY CONCLUSIONS (continued)
Optimization?
The optimization of the source parameters can be done with respect to the following factors of merit:
# [accepted yield]/PEDD => maximize {thinner targets and lower incident energy}
# Deposited energy density => minimize {larger e- beam dimensions}
# [pulse duration]/shock wave time propagation
=> maximize {choice of large enough pulse duration}
# Target rotation speed: => minimize
# Positron preaccelerator acceptance: => maximize
{choice of L-Band instead of S-Band}
Develop a precise quantitative approach for calculating the effects of thermal shock waves :finite elements
R.Chehab/LCWS10/Beijing